Sewage clarifier



W. H. GREEN SEWAGE CLARIFIER Nov. 28, 1944.

Filed July 24, 1941 'ciently clarifying sewage.

PatentedNov. 28, 1944 SEWAGE omnmmn Walter H. Green. Chicago, Ill.,aseignor to lnfileo Incorporated, a corporation of Delaware ApplicationJuly 24, 1941, Serial No 403,881

This invention relates to an improved method and apparatus for treatingsewage and liquid wastes. I

A primary object of the invention is to provide a method and apparatusforraw sewage clarification whereby the time required for clarificationis very greatly reduced and improved clarification obtained. At the sametime conditions contrary to septic action are maintained throughout thevolume of sewage undergoing clarification, thus avoiding objectionableodors and other undesirable conditions arising from long retention ofputrescible solids in ordinary sedimentation. Relative to this is thefurther object of retaining and concentrating the separated solids undercontraseptic conditions until a certain point is reached and thenquickly removing them from the process and apparatus so that they arenot subjected to any prolonged anaerobic action. In modern sewagetreatment clarificatio'n" is often thought of as sedimentation, but asused herein the term A still further object of this invention is toprovide a novel means for maintaining aerobic conditions in sewageduring clarification. It is well known that under the sedimentation ofthe prior art, which required a long holding time, sewage often .becameanaerobic and septic in character,

especially in the summer, and therefore, much more diflicult to clarify,as well as creating objectionable odors.

A, still further object of the invention is to proyide a method andapparatus for reducing the amount'or chemical reagent required for elli-Another important object of this invention is to provide a method andapparatus for removing from sewage colloidal and dissolved matter.Heretofore, it was possible to remove only the heavier suspended solids,commonly called settleable solids. By means ofmy invention I am not onlyenabled to remove a much larger percentage of the suspended solids but,in addition,

1 claims. (or. 210-2) to remove a substantial proportion of colloidaland dissolved solids.

A still further object of my invention is to reduce the biologicaloxygen demand, commonly spoken of as B. O. D.," of raw sewage beyond apoint obtainable by ordinary sedimentation Another object of myinvention is to provide a apparatus for removing suspended solids,colloidal matter and dissolved material in a :relatively small, singlestructure.

Other objects of the invention will be apparent from the description andclaims which follow.

Most present day sewage treatment plants utilize either what is commonlyknown as the activated sludge process or the trickling filter, process,in each of which purification is effected by a biological action.,Whichever of these is used, the biological step is preceded by asedimentation step to eflect removal of as great a portion of thesuspended solids as may be practical. The greater the removal of suchsolids the less the load on the biological step. thus decreasing thetreatment cost, increasing the throughput flow, enlarging the degree ofpurification, etc. A long retention time in the sedimentation basin andunder quiescent conditions is, in general, favorable to high solidsremoval, but with putrescible matter present, and particularly so inwarm weather, these conditions allow anaerobic condition to arise,particularly in the lower part of the basin and objectionable septicaction to take place. Furthermore, in such sedimentation tanks thesludge is deposited over a large floor area where it is subjected to afurther-considerable retention period under anaerobic conditions.Consequently, the size of the ettling basin must be less than isotherwise desirable, but usually sufilcient to provide a liquiddisplacement time of about one and one-half to two hours based on theaverage rate of flow over the day and retention of solids for prolongedperiods. The usual sedimentation tank removes only settleable solids andprovides a reduction of about half only 1 corresponding period in orderto secure a degree fiers, commonly called sludge, are usually rein aquiescent sedimentation chamber tends to cause development of anaerobic,or septic,'conditions therein which interferes with subsequent[biological purification. basins must be extremely large as they areusually constructed to provide a holding time of about two hours for theaverage fiow of sewage. Ordinarily, a domestic sewage plant is designedto handle'a fiow of sewage twice as large as the average daily rate.Such tanks are not only ex pensive but obviously they provideexceedingly long holding times during th period in which I the fiow ofsewage to be treated is low, thereby tending to give rise .to septicconditions. Only a portion, usually about half, of the suspended solidsare removed from the sewage.

(b) The sedimentation I,

This failure to completely clarify the liquid places a heavy burden onthe biological purification and consequently requires large and moreexpensive treatment facilities, and greater operating costs.

Many modifications have been made in details of the sewage treatmentbriefly outlined above in order to avoid the difficulties which arisefrom the character of the impurities present and the consequent greattendency for septic conditions to develop. However, such modificationsof the process leave untouched the prolonged holding of the sewage inlarge sedimentation chambers. My invention approaches the problem froman entirely new angle. The apparatus used by me is much smaller than anyheretofore known for like flows, as instead of sedimentation periods oftwo hours or upwards I can clarify the sewage in a period of a fewminutes. Also, by my invention I am able to remove a much greaterproportion of the solids in the first clarification step so that thefollowing treatment for purification of the effluent is much lessexpensive than heretofore known. Finally, I have found that by the useof my apparatus I am able to avoid the development of septic conditionsin the sewage during clarification.

Briefly, my invention comprises the concentration of the solids to aconsiderable degree while retaining them in'suspension in sewageundergoing treatment and, during this period, preventing or minimizingto a necessary degree septic conditions that would otherwise arise, bymeans hereinafter set forth. At the same time, I utilize the solidspresent to assist in the process, and then quickly separate and removethe solids from the process and apparatus. My invention furthercomprises apparatus in which and by which conditions necessary andfavorable to the process referred to may be established and maintainedand the process carried out.

The process of my invention comprises turbulently mixing raw sewage,preferably but not necessarily with an addition of a coagulatingreagent, with a major quantity of slurry containing solids separatedfrom previously treated sewage, continuously circulating the mixture ina flow of substantial turbulence and velocity during such treatment,mixing enough air in the circulating sewage to prevent the developmentof anaerobic conditions therein, withdrawing clarified sewage from theupper portion of the sewage and solids from the lower portion.

It will be evident that, by providing a turbulent In the preferredembodiment of my invention I provide for stirring the surface of thecirculating sewage to break up any scum formation and to increaseaeration to avoid the possibility of septic conditions developing duringtreatment. I have found that by means of my treatment ordinary sewagecan be clarified in a period of about thirty minutes or sometimes less,as compared to periods of one and one-half to two hours or more requiredfor clarification by sedimentation. It is evident, therefore, that myapparatus will be much smaller than the usual clarification tank andthat even if there should be no aeration of the sewage, there will beless likelihood of septicity than heretofore.

My invention should not be confused with the activated sludge" typetreatment in which clarified sewage liquor is aerated and agitated for aperiod of ordinarily about four to six hours to permit thoroughbiological purification of the liquor and the aggregation of microscopicparticles by biological means, which activated sludge" step is precededand followed by sedimentation to remove solids. My process relates tosewage clarification, and, as above indicated, requires only a fewminutes to complete this phase of the treatment. As I am concerned onlywith clarification, a long period, such as is required in "activatedsludge treatment, is not needed. In effect, my apparatus will besubstituted for the "clarifier of the prior art. My apparatus ordinarilywill be followed by the usual trickling filter or the activated sludgeaerator. In either event, it will be found that operations of the entireplant will be improved as the sewage liquor undergoing purificationcontains less pollution load and is inuch fresher than heretofore.

A preferred embodiment of the apparatus of the present invention isshown in the accompanying drawing which forms a part of thisspecification, and in which like reference characters in the severalfigures designate similar elements, and the process itself will beunderstood from the description and discussion of the apparatus takenwith other disclosures thereof herein.

Figure 1 is a vertical cross-sectional view of the preferred embodimentof my invention.

Figure 2 is a plan view of the apparatus shown in Figure 1 taken fromthe horizontal plane 2-4 of Figure 1.

The apparatus of the present invention includes any suitable tank, suchas a, cylindrical tank I, formed by a vertical wall II, and hopperbottom A clarified sewage effluent i3 is connected to an effluentlaunder I4 which is located adjacent the top l5 of th tank I0. I preferthat the bottom be formed with a steep slope although a small flatbottom I6 is permissible. This construction prevents undue accumulationof sludge, which would rapidly become septic if prolonged retention ofit were permitted. I also prefer to provide a pocket 'll for collectionof sludge adjacent the sloping walls I 2 of the tank. The pocket I1 canbe formed in any suitable manner, but I 'i prefer to use one formed by aconical baffle 18 in tank by any suitable means, such as brackets orlegs l9. The battle |8 should be so spaced as to permit ready access ofsolids from the tank, as by means of annular communication 20 and toprotect sludge concentrating or accumulating therein from'the turbulentconditions in the lower portion of the tank. Means should also be provided, as by an outlet 2|, for the upward rise of. A sludge outlet. 22

impeller. or other suitable propelling means, for

producing a circulation of considerable volume and velocity through thecirculating zone, to maintain the solids contained in the circulatingsewage in suspension. and to aid in coagulating colloidal or dissolvedmatter. Various arrangements of the partition and impeller will be obvi--ous to those skilled in the art, but I have used andprefer anarrangement such as illustratedin the drawing. The preferred arrangementof partition 25 consists of a conical hood 26 in spaced relationshipfrom the hopper bottom |2 of the tank and supported by any suitablemeans,such as legs 21. A vertical skirt 28 may be attached to the lowerrim 29 of the hood. The upper portion of the hood is open, as at 30, andcommunicates with an intermediate vertical cylinder 3|. The verticalcylinder 3|, at its upperend 32, communicates with an upwardly expandingor inverted conical chamber 33. The chamber 33 should extend above theliquid level in the apparatus as it is very desirable that the liquidbeing mixed and circulated through this chamber be prevented fromoverflowing into the upper portion of thetank Ill. Therefore, in thoseinstallations in which the size and shape of the tank l makes itdesirable to do so I may provide a short vertical cylinder 34 at theupper rim 35 ofthe upwardly expanding chamber 33. in order to raise thechamber above the liquid level of the apparatus without approaching tooclosely to the upper rim |"of the tank. I further provide a pluralityof-vertical draft tubes, such as 3' 31,

38, and 39, leading from adjacent the upper portion of the upwardlyexpandin chamber 33 down to the lower portion of the tank. Preferablyall not shown.

parts of the partition will be vertical or be steeply sloping, at anangle of not less than 45 degrees from horizontal, to prevent sludgefrom remaining thereon.

I Coaxially aligned within the flow space enclosed by the partition 25,I place a mixing and agitating apparatus 43 which" may be of anysuitable arrangement. I show a preferred'embodiment in the drawing.Briefly, it comprises ashaft 44, the lower end of which is journaled ina bearing 45, located in the bottom of the tank. 'The upper end of theshaft 44 is attached to reducing gears 45 which in turnare drivenby amotor 41. The reducer and motor should be placed above the liquid levelin the apparatus and can be supported by any suitable means, such as abridge 48. Aflixed to the shaft are impellers and agitators to provide aturbulent mixing within the" partition 25 and a circulationtherethrough, and through'the down draft tubes 36, 31, 38, and 39v ofrelatively high velocity. For this purpose I prefer to place an impeller49, rigidly aflixed to the shaft 44, within the intermediate cylinder3|, which provides communication between the hood 2'6 and the upwardlyexpanding chamber 33. I also prefer to place a larger impeller 50', alsorigidly afiixed to the shaft 44, within the upwardly expanding chamber33. At the liquid level in the apparatus I prefer to have a rake 5| foragitating the surface of the liquid in the upwardly expanding chamber33. This stirs air into the surface of the sewage undergoing treatment.Some air is absorbed by the circulating sewage and carried into thelower portion of the tank. In this way and also due to the repeated andrapid circulation, aerobic conditions are maintained throughout thevolume of sewage. The rake 5| maybe of any suitable construction. suchas a horizontal bar 52 rigidly aflixed to the shaft 44 with a pluralityof teeth 53 extending down into the liquid.

A raw sewage infiuent 51 discharges into the space enclosed by the hood26, as at 5B. A chemical feed line 59, likewise may, discharge into thespace enclosed by the hood, as at Bil. In some installations it may bedesirable to provide means,

. such as air line 6 provided with suitable diffusers,

such as nozzles 62, for the forced aeration of sewage in the lowerportion of the tank and supplied with compressed air by any suitablemeans, I have found in actual operation that in ordinary domesticsewages such aeration is not necessary if turbulent agitation andsuitable circulation is maintained. as above described. If desired, boththe necessary circulation and agitation as well as aeration can be hadby blowing in air in this way, and then the other means of agitationdescribed may be omitted.

The operation of the apparatus will. readily be understood. Raw sewageis discharged under the hood 26 through the influent line 51 andchemicals, if used, are introduced through the feed line 59. The motor41 is operated to cause the impellers to rotate at a speed sufficient toprovide a volume of flow of sewage through the mixing and circulatingspace defined by the partition 25 and the connected down draft tubes,several times the through put of sewage. It is important to pro-;

arated and retained from previously treated sewage as it flows upwardlythrough the partition 25. The major portion of the sewage issuing fromthe lower ends of the downdraft tubes 36,

31, as and as, will pass around and into the hood- 26 and upwardly withthe raw sewage. being introduced. However, an amount of treated sewageequivalent to the input of raw sewage will be displaced from the mouthsof the downdraft tubes and will rise upwardly in the tank I0. After aninitial period of operation it will be found that the retained solidscontained in the circulating sewage have altered in character, beingapparently dewatered to a considerable extent, denser.

and also aggregated to form some larger masses.

' Contrary to former teachings as to flocculating sewage, whichadvocated very mild-agitation with low agitator velocities and norecirculation of sewwa'rdly from the mouths of the downdraft tubes,

the rising water will carry relatively few particles upwardly out of theslurry. The solids will,

therefore, remain in suspension in the lower part of the tank as aslurry and the clarified water will quietly flow upwardly tothe top ofthe tank It and be withdrawn by the efliuent launder l4. Due toquiescent conditions in the protected space under the baille it, much ofthe solids content will deposit from the slurry therein to form a sludgeon the floor. The resulting thinner slurry will rise through the opentop 2! of the baflle ill to be replaced by slurry entering through thespace around the lower edge of the baffle. In this way there is acontinuous mild circulation in the pocket and solids will be depositedin the pocket I1 from which they may be removed either continuously orintermittently through a line 22. Thus there is a very short retentiontime for solids outside the aerobic or contra septic conditions of thecirculation.

From my experiments and experience so far, I have found that the averagetime of retention of the sewage in the circulation zone should be atleast about ten minutes, and fifteen minutes is apparently somewhatbetter. A longer time of retention, up to half an hour and more is notharmfulif contra septic conditions are maintained, but there is noapparent benefit with ordinary domestic sewage beyond about fifteenminutes. As noted before, the rate of circulation desirably is such asto turn over the whole volume in the circulation zone once in about fiveminutes. This is not a rigid time but too slow and and quiescentcirculation is unfavorable to the formation of the right sort of slurryfor good clarification and also unfavorable for aeration. The time ofretention above the lower end of the downdraft tubes is not important inand of itself and will be largely determined by construction conditionsor features. The important thing is the vertical rate of rise asdetermined by the area between the inner structure and the tank wall andthe throughput, and the apparatus is preferably constructed to give anormal upflow rate of about two gallons per square foot per minute.Lower rates are not harmful in general and I have used substantiallyhigher rates. It is, therefore, possible to clarify sewage in a periodof about twenty to forty minutes in lieu of a period of two hoursrequired for sedimentation.

Experience with the process and apparatus herein defined has illustratedthat under ordinary conditions sewage undergoing treatment in myapparatus will not become septic. This I believe to be due to twofactors: first, the fact that the period required for clarification ismuch shorter than the period required for settling in the old style ofsedimentation tanks' and the prolonged anaerobic storage of sludgetherein; and second, rapid circulation of the sewage undergoingtreatment. maintains aerobic conditions throughout the whole volume ofthe sewage. I

have found that the rake 5i may be omitted without septic conditionsarising. I prefer, however, to have the circulation of sewage undergoingtreatment extend upwardly so as to reach the air and to "provide therake 5| to mix some air into the sewage undergoing treatment. It .is tobe understood that the overcoming of septic action by aeration, whileadvantageous and preferred,

is not essential to the success of the process.

Antiseptic reagents, such as, for example, chlorine, may be utilized-toinhibit undesirable conditions for the time of retention. If lime beused as a coagulating or precipitating. reagent an amount may be addedto raise the pH value to a point unfavorable to septic growth.

The degree of purification obtained by my process and apparatus comparesfavorably with other processes, such as chemical treatment withsedimentation. I have found that without any coagulating reagent asolids reduction of fifty percent or more with corresponding B. O. D.reduction is had, this being about the same as with ordinary two hoursedimentation. By the use of coagulating reagents in suitable doses.

higher reduction can be had and by such means I haveobtained suspendedsolids reduction up to ninety percent or more within the same timeperiod. In fact, higher rates of flow may be used with coagulation andthe time period proportionately shortened. I have also found that withmy invention the amount of reagent necessary can be materially reduced.

I have found that water and solids separate very readily within a shortdistance from the draft tubes. Usually thereis a sharply defined line ofdemarcation between the slurry in the bottom of the tank and theclarified sewage above. This slurry interface, as it is-sometimescalled, is often so sharp as to require only a small fraction of an inchto pass from clear sewage into dense slurry. I find that betteroperating conditions are secured if the slurry interface is maintainedat the level of the intermediate cylinder 3l. Apparent1y, it ispreferable to maintain the circulating and agitating mechanism 43 can beremoved and in place thereof a plurality of compressed air jets 62discharge within the hood 26.

' tion and rapid circulation of the sewage and slurry are sufficient inand by themselves to provide for the conditioning of sewage solids tank.It ,is,'therefore, entirely feasible with my apparatus to rapidly andefficiently clarify raw sewage without the use of a chemical reagent.

As used herein, the term sewage refers to domestic sewage, tradewastes,-polluted liquids and other waste liquids. The term slurry asused herein refers to a suspension of sewage being treated and aquantity of solids accumulated from previously treated sewage,preferably while in a suspended condition, several times greater thanthat in, or formed in, an equal quantity of sewage entering to betreated.

Manifestly, many modifications and variations of the inventionhereinbefore set forthmay be made by persons skilled in the art withoutdeparting from the spirit and scope thereof.

I claim:

l. Sewage clarification apparatus comprising a tank, a verticallyextending annular partition structure within said tank, said partitionstructure being spaced above the floor of said tank and enclosing acentral mixing chamber, said mixing chamber and-the outer annular spacein said tank between the wall thereof and said partition being in opencommunication below said partition structure, conduit means leading froman upper part of the mixing chamber into an intermediate part of theouter annular space, energy delivery means in said mixing chamber. saidmeans being so positioned that the energy zone below. said mixing space,an inlet into said sheltered subsidence zone from the lower portion ofsaid mixing space, an outlet from said sheltered subsidence zone intosaid mixing space, and a waste discharge outlet from said shelteredsubsidence zone.

3. Sewage clarification apparatus comprising a tank, a partitionstructure extending vertically in said tank from a level in the lowerportion thereof to above the normal liquid level therein and separatingthe tank into two laterally adjacent spaces in open communication onewith the other under. the lower edge of said partition,

conduit means having an inlet from the upper portion of the first ofsaid spaces and discharging into the second of said spaces at a leveladjacent the lower end thereof, said partition and said conduit being soarranged as to leave a quiescent zone in the upper part of said secondspace, saidfirst space and said conduit means together with the spacebelow the partition structure forming a mixing and circulating zonethrough which sewage undergoing treatment is passed in a cyclic path, a,raw sewage inlet for delivering-incoming sewage into the mixing andcirculating zone, a mechanically'driven stream projecting impellerwithin said mixing and cirdelivered thereby will cause vertical motionof.

liquid in said chamber, a source ofenergy out- .side said chamber, meansoperatively'connecting said source of energy to said energy deliverymeans, inlet means for inflow of raw sewage discharging into said mixingchamber, an outlet for clarified sewage from the upper part of saidouter annular space, a second partition structure below the mixingchamber and so positioned relative to a wall of said tank as to formtherewith a sheltered subsidence zone below said mixing chamber, saidsubsidence zone having an inlet and outlet communicating with saidmixing said chamber together forming a mixing space extending from thelower portion of the tank to above the normal liquid level therein, -aconduit opening from the upper partsof the mixing space and extendingdownwardly to a lower level of said tank, a power driven liquidpropeller of such size and so positioned in said mixing space as onoperation thereof to cause an upward flow through said mixing space of amagnitude of at least about twice that of the normal inflow ofunclarified sewage, means for delivering raw sewage into said mixingspace, a clarified sewage outlet from the upper portion of the said tankoutside of said mixing space, a second partition in said tank below saidhood, said second partition being so shaped and so positioned as to formjointly with a wall of said tank a sheltered subsidence culating zoneand so constructed and arranged as to impart turbulent mixing and arapid flow to liquid in, said mixing and circulating zone, an outletfrom the top of the quiescent zone, a second partition structure belowthe mixing and circulating zone so shaped and so positioned relative toa wall of the tank as to form therewith a sheltered subsidence zone anda passage between said mixing and circulating zone and said shelteredsubsidence zone for inflow of sews age into said sheltered subsidencezone, a passage for outflow of partially clarified sewage from saidsheltered subsidence zone, and a solids discharge from the lower portionof said subsidence zone.

4. A process of clarifying sewage in a tank, whichcomprises the steps ofmaintaining in said tank a body of sewage to be clarified, maintaining apool of slurry containing uns-edimented solids separated and accumulatedfrom previously treated sewage and kept suspended in sewage undergoingtreatment in the lower portion of said body of sewage, maintaining acontinuous and essentially vertical circulation of slurry ofconsiderable volume leading from said pool of slurry through a confinedmixing and -reaction zone and back into said pool, discharging rawsewage into said circulating slurry, imparting mechanical impellingenergy in addition to any energy imparted by the incoming sewage to saidcirculating slurry to cause mixing of sewage and circulating slurry andto maintain said circulation of slurry, displacing clarified sewageupwardly from the upper surface of said pool of slurry into a clarifiedsewage space, withdrawing clarified sewage from adjacent the top of saidclarified sewage space, continuously passing a'portion of the slurryinto a quiescent subsidence zone located within said pool of slurry andopening into the mixing and reaction zone, depositing solids from slurryin said quiescent subsidence zone, returning partially clarified sewagefrom said quiescent subsidence zone to said pool of slurry, andwithdrawing deposited solids from the lower portion of said subsidencezone.

5. A sewage clarification process comprising the steps of confiningv abody of sewage for a period in a treatment zone. accumulating within thelower portion of said body 01 sewage a suspension of solids separatedfrom previously treated sewage, maintaining a continuous majorrecirculation embracing substantially all or the suspension, confiningsaid'recirculation to and through a cyclic path which extends upwardlyfrom and returns to the lower portion of said zone, maintaining a,quiescent zonein an upper portion of said treatment zone, flowing rawsewage into. said recirculating suspension and thereby displacing acorresponding amount of suspension therefrom into said quiescent zone,separating and withdrawing clarified sewage from the contents of saidquiescent zone and so eflecting a further concentration of solids insaid suspension, passing a minor flow out of said recirculatingsuspension slowly into and through still another and quiescent lowerportion of said treatment zone and back into said recirculation,

depositing solids from the suspension passing through said quiescentportion, and discharging such deposited solids to waste.

6. The process of claim 5 wherein the recirculation is caused by,and'the energy required for maintaining said recirculation is suppliedby, discharging compressed air into said recirculating suspension.

7. Sewage clarification apparatus comprising a tank, a small partitionstructure adjacent the bottom of the tank so constructed and arranged inrelation to a wall of the tank asrto'form a sheltered subsidence zone inthe lower portion I of the tank having an inlet and an outletcommunicating with the lower portion of said tank, a large annularpartition structure above said small partition and extending verticallyin said tank to above the normal liquid level therein, said partitionforming a central space and an outer annular space in said tank, conduitmeans having an inlet from the upper portion of the central space anddischarging into the lower part of the outer annular space, a raw sewageinlet for delivering incoming sewage into said rapid vertical how tosewage inside said space, a clarified sewage outlet from the upperportion" of said outer annular space, and a solids 'discharge from thelower portion of said subsidence zone.

WALTER H. GREEN.

